1. Field of the Invention
The present invention relates to a passive vehicle occupant restraint, such as an air bag, for restraining a vehicle occupant in the event of an impact on a vehicle, and more specifically to a technique for controlling activation of such a passive vehicle occupant restraint.
2. Description of the Prior Art
An apparatus for controlling squib ignition in an air bag is one of known activation control apparatuses for controlling activation of passive vehicle occupant restraints. In the air bag, a squib ignites a gas-generating agent in an inflator on impact, and a gas accordingly evolves from the inflator to inflate a bag and protect a vehicle occupant from the impact of a collision.
The apparatus for controlling squib ignition in an air bag typically measures the impact on the vehicle as a deceleration by an acceleration sensor, calculates a function of the measured deceleration, compares the calculated value of the function with a preset threshold value, and controls the squib ignition based on the result of comparison. The acceleration sensor is disposed at a predetermined position in the vehicle, generally on a floor tunnel in the vehicle. In the description hereinafter, the acceleration sensor mounted on the floor tunnel is referred to as the floor sensor.
The threshold value is set to be greater than the maximum among the values of the function calculated from the decelerations measured by the floor sensor when impacts applied to the vehicle have such a degree that does not require activation of the air bag.
In the conventional activation control apparatus for a passive vehicle occupant restraint, the impact applied to the vehicle is detected only by means of the floor sensor, and activation of the passive vehicle occupant restraint is controlled based on the result of detection. The following problems arise in this conventional structure.
Collisions of the vehicle are classified into several types by the condition and the direction of the collision and the type of the object against which the vehicle collides; that is, a head-on collision, an oblique collision, a pole collision, an offset collision, and an under-ride collision as shown in FIG. 27. In the case of a head-on collision, the vehicle receives an impact of the collision by left and right side members thereof, so that an extremely large deceleration arises on the floor tunnel with the floor sensor mounted thereon within a predetermined time period after the collision. In the case of collisions other than the head-on collision, however, the vehicle does not receive the impact of the collision in such a manner, so that no such a large deceleration arises on the floor tunnel within the predetermined time period after the collision.
Namely the floor sensor has higher sensitivity of detection of the impact within a predetermined time period after the collision in the case of a head-on collision, than in the case of the other collisions.
The threshold value is thus set mainly based on the deceleration measured in the case of a head-on collision. More concretely the threshold value is set based on the function calculated from the deceleration measured by the floor sensor when a head-on collision applies an impact of such a degree that does not require activation of the air bag to the vehicle.
This method for setting the threshold value based on the deceleration measured in the case of a head-on collision gives a relatively large threshold value. In the case of collisions other than the head-on collision, the floor sensor has relatively low sensitivity of detection of the impact within a predetermined time period after the collision as mentioned above. A DSP (digital signal processor) is accordingly used for Fourier transform of the deceleration signal obtained in the event of the collision to characteristics of a specific frequency component. In the case of the other collisions (including an offset collision), the impact is detected based on the characteristics of the specific frequency component.
This technique requires the DSP and the other related devices as well as a high-performance computer, which undesirably increases the cost.